Expanded graphite sheets are commonly fabricated in the following manner. Natural graphite, pyrolytic graphite, kish graphite, or the like is treated with a mixture solution of sulfuric acid, nitric acid, or the like, washed with water, and then dried. Thereafter, the resultant material is subjected to an expansion process in an expansion furnace at about 1000° C., and this is made in a sheet form by pressure-rolling using rollers or the like. The expanded graphite sheets have excellent heat resistance and excellent gas-liquid impermeability. For this reason, they are used for packings, gaskets, mold release sheets in hot pressing, linings for carbon crucibles, heat dissipating members for electronic devices, and the like.
Here, if the expanded graphite sheet contains ash in a large amount, the mating materials such as metal may be damaged in the case where the expanded graphite sheet is used as a packing, a gasket, or the like. In the case where it is used for mold release in hot pressing or the like, it is possible that the ash may be transferred to the materials to be molded and bumps and dents, or patterns may be formed.
In view of this, a technique has been disclosed of obtaining a high-purity expanded graphite sheet that can reduce the content of impurities by subjecting an expanded graphite sheet to a refining process under a halogen gas atmosphere at 800° C. or higher, and this material is also used for a semiconductor manufacturing process (see Patent Document 1 indicated below).